The in vitro activities of piperacillin, azlocillin, mezlocillin, sulbenicillin and ticarcillin were compared with those of carbenicillin using 88 clinical isolates of Pseudomonas aeruginosa. The minimum inhibitory concentrations (MIC) and the minimum bactericidal concentrations (MBC) were determined by standard techniques. The MIC for 90% of the strains was 7.5 mg/l for piperacillin, 10.0 mg/l for azlocillin, 26.5 mg/l for mezlocillin, 48.4 mg/l for sulbenicillin, 50.0 mg/l for ticarcillin and more than 100 mg/l for carbenicillin. The MBC/MIC ratio was 1.3 for piperacillin, 1.9 for ticarcillin, 2.1 for sulbenicillin, 3.3 for mezlocillin and 4.5 for azlocillin. The susceptibilities of the same strains to four aminoglycosides were tested. The MIC for 90% of the strains was 0.3 mg/l for sisomicin and tobramycin, 1.5 mg/l for amikacin, and 2.2 mg/l for gentamicin. The effect of combining piperacillin, azlocillin and mezlocillin with gentamicin, tobramycin, sisomicin and amikacin was studied using checkerboard titration. The highest degrees of synergy were found with the combinations of piperacillin and an aminoglycoside. Strong potentiation was observed in 85% of the strains with piperacillin - sisomicin and in 50% with piperacillin - gentamicin. The synergistic effects of azlocillin and mezlocillin in combination with an aminoglycoside (observed in 30-65% of the strains) were for the most part moderate or slight. No antagonism was observed.